ORIGINAL ARTICLE Effects of high-relief structures on cold temperate fish assemblages: A field experiment DAN WILHELMSSON 1 , SALEH A. S. YAHYA 1,2 & MARCUS C. O ¨ HMAN 1 1 Department of Zoology, Stockholm University, S-106 91 Stockholm, Sweden, 2 Institute of Marine Sciences, University of Dar es Salaam, P.O. Box 668, Zanzibar, Tanzania Abstract High-relief structures may influence the abundance and diversity of reef-associated fish. We conducted a field experiment to investigate whether the presence of vertical structures (PVC pipes) affects fish communities on artificial reefs. The effect of the height of the structures (1 and 3 m) was also tested. Furthermore, the effects on fish of placing artificial reefs on otherwise featureless bottoms were quantified. Algal and macro-invertebrate colonization of the reefs was also recorded. The experiment was carried out on the west coast of Sweden over a period of 1 year. The vertical structures had a positive effect on fish abundance but not on diversity. The height of the structures did not, however, influence the fish communities. Natural as well as urban vertical structures on the seafloor could have a positive effect on local fish abundance. The positive effects of artificial reefs on total fish abundance and diversity were immediate. Of the 10 species recorded, two, the black goby Gobius niger and the goldsinny wrasse Ctenolabrus rupestris , dominated over the whole survey period. There were significant temporal differences in fish abundance, and diversity increased with time. Key words: Artificial reefs, biodiversity, human disturbance, offshore wind power, reef profile, Sweden Introduction An enhanced understanding of the factors regulating fish assemblages is of paramount importance in conservation efforts as well as in fisheries manage- ment. For example, fish communities that proliferate in association with the seabed are influenced by bottom composition and structure. This has been noted in a number of studies in tropical (Jennings et al. 1996; Chabanet et al. 1997; O ¨ hman & Rajasuriya 1998; Garpe & O ¨ hman 2003) and temperate waters (Choat & Ayling 1987; Jones 1988; Anderson 1994), whereas there is a paucity of such information from cold temperate seas (but see for example Pihl et al. 1994; Pihl & Wennhage 2002). Different habitat variables, such as structural complexity (Anderson 1994; O ¨ hman et al. 1998), substrate composition and habitat heterogeneity (e.g. Risk 1972; Luckhurst & Luckhurst 1978; Bouchon-Navaro et al. 1985; Chabanet et al. 1997) have been shown to be important in regulating demersal (bottom-dwelling) fish assemblages. Although the results are not conclusive (see for example Parrish & Boland 2004; Gratwicke & Speight 2005a), the vertical relief of the substrate is another factor that has been considered to be of importance for the abundance and diversity of fish. For example, fish abundance was positively corre- lated with the height of bottom structures (relief height) in a study by Gratwicke & Speight (2005b). Fish species richness and total abundance were shown to be saturating functions of reef height and percentage vertical surface (Patton et al. 1985). The diversity of heights represented on a reef (‘‘vertical diversity’’) has been suggested to influence fish community structure (Helvey & Smith 1985). Also, vertical relief was one of the most important habitat factors influencing fish community structure on some coral reefs in the Caribbean (Lara & Gonza ´les 1998). Taxon-specific studies on natural bottoms in tropical and temperate waters have further shown that, for example, juvenile cod (Gadus morhua ), rockfish (Sebastes spp.), serranids (Serranidae), and Correspondence: D. Wilhelmsson, Department of Zoology, Stockholm University, S-10691 Stockholm, Sweden. E-mail: dan. wilhelmsson@zoologi.su.se Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory, University of Copenhagen, Denmark Marine Biology Research, 2006; 2: 136  147 (Accepted 16 February 2006; Printed 6 June 2006) ISSN 1745-1000 print/ISSN 1745-1019 online # 2006 Taylor & Francis DOI: 10.1080/17451000600684359